This project is designed to study the relative lack of repair of radiation injury from fetal exposure which persists in the adult mouse. Pregnant female mice will be given a priming exposure of X-irradiation at various times during gestation. The radiosensitivity of the following tissues and organs will then be measured at various ages throughout postnatal life: Hematopoietic system, gastrointestinal tract, spleen, thymus, and small intestine. A comparison of radiation sensitivity of these systems between mice irradiated in utero and unexposed controls will provide measurement of irreparable injury from fetal irradiation.